Gregg B has said it very well. You need to train your fingers to go where your ears recognize the right intonation. Fiddles are entirely chromatic, where guitars and other fretted instruments are not. Small adjustments in your fingers allow you to play in any key. Learning to play a fiddle with frets will not teach you how to do this. It does require a good ear, and the ear can be trained. And without frets you can do all those neat slides that make the fiddle more like the human voice.
"What about vibrato?" Don't worry about it. It isn't necessary with traditional fiddling. It may develop in time, it may come naturally, but it's not worth worrying about.